Electrodes are an essential component for charge storage and delivery in high performance electrochemical energy storage devices. High performance electrodes are components for electrochemical devices including, for example, ultracapacitors, batteries, fuel cells, sensors, and photoelectrochemical solar cells. Broadly, electrochemical energy storage devices are used in utility, transportation, electronics, medical, and defense applications. More specifically, electrochemical energy storage devices have applications for: emergency backup power for electrical transmission and distribution systems; electric, hybrid-electric, and plug-in hybrid vehicles; notebook computers; cellular telephones; pagers; video cameras; hand-held tools; portable defibrillators; drug delivery units; neurological stimulators; specialized mobile power applications; unmanned aerial vehicles; spacecraft probes; and missile systems.
Ultracapacitors (also referred to as supercapacitors, electrical double-layer capacitors, or electrochemical capacitors), electrochemical are energy storage devices that combine the high-energy storage potential of batteries with the high-energy transfer rate and high recharging capabilities of capacitors. Ultracapacitors can have energy densities orders of magnitude greater than conventional capacitors and power densities orders of magnitude greater than conventional batteries. Generally, ultracapacitors have energy densities in the range of about 1 to about 10 Wh/kg, which is about one-tenth of that of secondary batteries, which have energy densities of about 20 to about 100 Wh/kg, and power densities in the range of about 1,000 to about 2,000 W/kg, which is about ten times higher than those of secondary batteries, which have power densities in the range of about 50 to about 200 W/kg.
Energy storage in ultracapacitors can be either Faradic or non-Faradic. One common type of Faradic ultracapacitor is an oxidative/reductive ultracapacitor, the Faradic capacitor transfers electrons across an electrode interface. Faradic ultracapacitors are based on mixed metal oxides, such as ruthenium dioxide and other transition metal oxides. For reasons of high cost, scarcity, and toxicity of suitable metal oxides, Faradic ultracapacitors are generally not preferred in most applications.
In non-Faradic ultracapacitors, electron transfer does not take place across an electrode interface, instead electric charge and energy are electrostatically stored. That is, positive and negative electrostatic charges accumulate on the electrodes at the electrode-electrolyte interface. Electrical energy is stored as an electrostatic force in form of a charge separation in the electric double layer between an ionically conducting electrolyte and the electrode.
In Faradic and non-Faradic capacitor systems, capacitance depends on the characteristics and properties of the electrode material. The electrode material preferably has electrically conducting properties, a porous structure, or both. The porous structure (that is, pore size, pore size distribution, and pore volume fraction) provides a large surface area for the development of an electrical double layer for non-Faradic static charge storage or for reversible chemical oxidation-reduction reactions for Faradic capacitance.
In commercial ultracapacitors, the electrode material comprises an activated carbon. FIG. 1 depicts an activated carbon electrode 90 of the prior art, comprising a metal current collector 101 and a carbon coating 103. The carbon coating 103 comprises a mixture of carbon black 105 and activated carbon 107 particles dispersed in a polymer matrix 111. FIG. 2 depicts a scanning electron microscope image of the carbon coating 103 comprising activated carbon 107 and carbon black 105 particles. The carbon black particles 105 form carbon black aggregates 109 (see FIG. 3). The carbon black aggregates 109 are positioned between and in physical contact with one or more activated carbon particles 107.
The charge storage capacity of the electrode 90 varies with the type of carbon (that is, carbon black and activated carbon) and the accessibility of the carbon to the electrolyte. Poor electrolyte accessibility to the carbon is a primary reason for decreased capacitance.
Therefore, a need exists for electrodes capable of providing high energy and power densities, as well as, longer cycle lives and safer operating conditions. That is, a need exists for electrodes that more effectively and efficiency store and/or deliver charge.